1. Field of the Invention
The present invention pertains to crop harvesting machines and more particularly to such machines comprising apparatus for detecting the presence of undesirable objects in the machine.
2. Description of the Prior Art
A variety of objects such as stones, pieces of wood, plastics and metal may lie in the path of a harvesting machine which cuts or picks up crop and these are liable to cause damage to the crop processing means within the machine. Harvesting machines which are particularly subject to such damages are forage harvesters. The crop processing means in a forage harvester comprise a cutterhead and associated shearbar which are of a precision character and have fine clearances and are buried in the body of the machine, whereby ready access thereto is denied. A cutterhead normally operates at a high rotational speed and has considerable inertia so that severe damage can arise if foreign objects reach the cutterhead. Pieces of wood are not generally a severe hazard. Stones are more so but generally splinter although they cause chipped blades on the cutterhead. Plastics and nonferrous metals are rare, and in any case relatively soft. Thus the main risk arises from ferrous metal objects which are both numerous and harmful. Such undesirable objects may be whole or broken parts that have become detached from machines, tools which have been left in the field or which have bounced out of moving machines, hitch pins, fencing stakes and scrap dumped by intruders. The high present-day level of mechanisation, together with the pressures on a diminishing labour force, have probably led to an increased risk of ferrous scrap in all areas, augmented near towns by habits of waste disposal.
Whether or not they give rise to damage to a machine, metal objects passing through will find their way into the crop processed by the machine and may ultimately injure livestock fed with it.
Detectors in general and metal detectors employed on forage harvesters are already known in the art. However, prior art devices have one or more inherent disadvantages rendering them of little value in certain applications, especially in the reliable protection of expensive crop harvesting machinery. Known metal detectors often operate at high excitation frequencies and are susceptible to the presence of moisture, various types of vegetation, vibration, or the like. Therefore, they impose on the machine operator the serious burden of continuous monitoring and adjustment or calibration of the detection device. Even at relative low excitation frequencies, such detectors are adversely affected to a significant extent by an over-extensive scanning area, thereby rendering them unusable for use where high accuracy, reliability and freedom from generation of false alarms is demanded. Additionally, prior art detection devices are located at positions which, for one reason or another, may fail adequately to detect the presence of incoming foreign objects. An example of a detection device located at a position within a crop harvesting machine, which has been found less than satisfactory, is illustrated in the U.S. Pat. No. 3,757,501 which teaches the use of a detection device mounted in the floor plate of the header of a harvesting machine. With such a machine, it has been found that undesirable objects located on top of the incoming mat of crop material can go undetected. Furthermore, this location of the detection device creates the situation where each alternative attachment for the machine has to incorporate a detection device, rather than it being standard equipment in the body of the forage harvester, which increases the cost of the attachments.
Further, it is also known to use drive arresting means in combination with metal detectors on forage harvesters such that upon detection of an undesirable object, the drive to a crop feeder mechanism is stopped. Thereupon, a safety clutch in the drive train becomes operative. Hence the movement of the feeder mechanism is stopped abruptly which is desirable in order to prevent the passage of the detected undesirable object further into the machine. The operator can then reverse the feeder drive mechanism to remove the undesirable object. However, this abrupt arresting of the drive to the feeder mechanism may be harmful to the drive means itself. Also, as long as the operator takes no action, the safety clutch continues to slip and prolonged slipping is undesirable as it tends to result in overheating. Furthermore, after the drive to the feeder mechanism has been stopped by the operation of the detection device, it could happen that, rather than reversing the drive to the feeder mechanism to expel the undesirable object from the machine, the operator re-engages the feeder mechanism drive in the normal working direction by mistake. This, of course, would result in the undesirable object being fed to the cutterhead or other crop processing means and defeat the object of providing the detection device. Such an arrangement is disclosed in U.S. Pat. No. 3,959,953.
In still another known arrangement, a metal detector on a forage harvester has been combined with a feeder drive arresting device and an electric actuator which starts operating as soon as an undesirable object is detected. The electric actuator is coupled to a slip clutch mechanism in the drive train for the feeder mechanism and is operative upon detection of an undesirable object progressively to disengage the clutch mechanism. Thus this arrangement also gives rise to possible overheating and shock load problems and also makes it possible for the normal drive to be re-established before the detected object has been removed.